It is common practice to create a zonal isolation in an underground wellbore by inserting an inflatable elastomeric plug or packer in the wellbore.
If the wellbore is an uncased section of an underground borehole then the expanded plug or packer may exert a high radial force on the surrounding underground formation, thereby lowering the compressive hoop stresses in the formation such that fractures may be initiated in the formation adjacent to the plug or packer.
It is known from U.S. Pat. No. 5,623,993 to insert an expandable packer in a wellbore such that the impact on the compressive hoop stresses in the surrounding formation is limited. The packer is equipped with a water drainage conduit and granular material is deposited on top of the packer so that water will drain down through the matrix of granular material, thereby enhancing the packing density thereof. If subsequently a treatment and/or fracturing fluid is injected into the formation surrounding the borehole section above the packer, then the compacted plug of granular material transfers at least part of the axial load, which is due to the pressure differential over the pack to the inner surface of the wellbore along the interval packed with granules and thereby distributes the related radial force over a longer distance along a longitudinal axis of the wellbore, so that the risk of fracturing of the formation surrounding the inflated packer and adjacent compacted plug of granular material is inhibited.
The inflatable packer known from this prior art reference is only suitable for use in a wellbore region below the target section into which fluid is to be injected into the formation and is not suitable for use in irregularly shaped wellbores, such as an elliptically shaped borehole or a borehole with washouts, or for use in high temperature regions, such as in geothermal wells, since conventional inflatable packers comprise elastomeric materials that disintegrate at high temperatures.
U.S. Pat. Nos. 3,134,440; 3,623,550 and 4,423,783 disclose expandable well packers which comprise an umbrella-shaped frame which is expanded downhole to provide a barrier on top of which granular material, such as marbles, pea gravel and/or cement, is deposited to provide a fluid tight seal in the well. The known umbrella-shaped frame can conform to an irregular or unround wellbore to a limited extent, but is not configured to compact the granular material, so that the plug is only loosely set and may not penetrate into washouts and/or fractures in the surrounding formation.
U.S. Pat. No. 3,866,681 discloses a well packer wherein a granular packer is created on top of a doughnut device which is arranged around a slurry injection tubing and which comprises slurry transport channels with one way check valves such that a slurry can be injected down through the tubing and then up through the doughnut device into the annulus above the device where an annular matrix of granular material is induced to settle above the doughnut device.
Each of the known zonal isolations systems is configured to set a granular plug on top of an expandable barrier so that they can only be used to isolate a wellbore section below a target section.
It is an object of the present invention to provide a method for zonal isolation in a wellbore, which can be used to provide a zonal isolation between a target section and a wellbore section between a target section and a wellhead.
It is a further object of the present invention to provide a method for zonal isolation in a wellbore which is suitable for use in irregularly shaped wellbores and/or at high temperatures and which only exerts a limited radial force per unit length on the formation surrounding the wellbore, the risk of formation fracturing or weakening adjacent to the zonal isolation region.
It is a further object of the present invention to provide a method for zonal isolation between a target zone and a wellhead such that the length of the granular zonal isolation plug zone can be selected such that an elongate plug can be placed and the pressure differential can be distributed over a long longitudinal interval of the wellbore such that the risk of fluid bypassing via the formation surrounding the plug is reduced and that the pressure gradient profile along the length of the plug can be adjusted to the strength and other physical properties of the formation surrounding the plug.
It is a further objective of the present invention to provide a method for creating a zonal isolation, which can be easily removed or replaced to carry out a sequence of stimulation, fracturing or injection operations at different sections within a given well.